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Aeronautical Engineering/Control surface/yoke movement in turbulent air


QUESTION: What I'm trying to figure out is if in case of disconnected surface, the surface would assume an unfair position i.e. considerable deflected towards one extremity. Also, for the same disconnected surface, if gusts would move it considerable. I don't understand about what actuator forces you talked cause I wonder about a mechanical reversible system, not hyd actuated or other complex system.

ANSWER: By actuator forces I mean the friction and inertia in the control system plus the force provided by the pilot on the stick. If a control surface were disconnected I don't think it would move to an extreme position because the aerodynamic forces (float and restore discussed previously) would restrict the motion. But if while disconnected a gust hit, the control surface could be forced to an extreme position depending on the strength of the gust.

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QUESTION: Ok, not I got it.

"But if while disconnected a gust hit, the control surface could be forced to an extreme position depending on the strength of the gust."
This is where I don't get, if in normal use i.e. normal operation, there is no noticeable movement of the surface due to gusts, why there would be noticeable movement in the disconnected case? I can say the friction and inertia are neglijeable, because there is no difference moving the surfaces even on ground, and inflight, any feel pressure is sent directly to the yoke, there is no considerable difference.

I didnít say there would be no noticeable movement of a control surface due to gusts, I said it is unlikely that the control surface would move to an extreme position unless disconnected.  A connected control surface will move when hit with a gust, but the pilot should be able to stop it pretty quickly because he has a large moment arm and should have both hands or both feet on the controls.

You said that the control surface moves easily when moved by hand. I assume you pushed on the control surface trailing edge with no one in the cockpit. But the aerodynamic center of pressure is not at the trailing edge but much nearer to the hinge line. Short moment arm. Try your experiment again by pushing near 25% of the chord with someone in the cockpit putting normal resistance on the yoke. You should find it difficult to move to an extreme position.

Another thing to consider is how gusts act on a control surface. Assume the maximum range of the control surface is + and - 30 deg and you are flying with it set at +15 deg. A headwind gust would increase the velocity over the surface and force the trailing edge upward. If the pilot is not alert, the control surface may overshoot 0 deg. But then the aerodynamic forces are greater in the other direction  and will force the trailing edge back down. This will continue until the control surface stabilizes near 0 deg. If the gust was not a headwind but an extremely strong updraft that hit one wing, and the pilot had his hands off the yoke - yes, the aileron could move to an extreme position. That seems unlikely to me, but I canít say it canít happen. I assume if the updraft hit both ailerons equally the cross connection would prevent their movement.

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Paul Soderman


Aeronautics, Aerodynamics, Fluid Mechanics, Aeroacoustics, Noise Control, Muffler Design, Wind Tunnel Research.... I know nothing about India - do not ask about schools, jobs, application requirements, career choices, etc. for India. Please, no text message verbiage; I prefer full words in full sentences. Thanks.


38 years as research engineer at NASA


B.S. and M.S. Aeronautical Engineering - U. of Washington, Graduate work Standford U.

Awards and Honors
AIAA Associate Fellow (American Institute of Aeronautics and Astronautics)

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